With the increase in information capacity associated with the growth of the Internet and LANs (Local Area Networks), development of optical interconnection techniques employing optical signal have proceeded in various applications including not only telecommunication applications such as trunk lines and access systems but also short-distance signal transmission between or within boards in routers or servers. In such applications, an optical waveguide is preferably employed as an optical transmission line, since the optical waveguide attains higher wiring flexibility and higher wiring density, as compared with optical fiber. Among the materials of optical waveguides, a low-cost polymer material having high processability is a promising candidate. Specifically, since a polymer optical waveguide is employed for optical signal transmission between or within boards in routers and servers, an optical-electrical composite board on which the polymer optical waveguide is mounted in combination with an electric wiring board has been developed. As a method for producing the optical-electrical composite board, a light-exposure/development technique, which is generally employed in production of resist for printed wiring boards, is advantageous from the viewpoints of productivity and cost (see, for example, Patent Document 1).
Currently, optical waveguides employed in optical-electrical composite boards are required to meet wiring density demand of 250 μm-pitch (line & space of 50 μm:200 μm) in consideration of connection with a commercial optical element array; specifically, a VCSEL (Vertical Cavity Surface Emitting Laser) array, a PD (Photo Diode) array, or a multi-core ribbon optical fiber.
However, wiring density is thought to increase more and more in the future, and the aforementioned 250 μm pitch will be narrower. In order to keep pace with the trend, demand will arise for optical wiring of higher resolution.    [Patent Document 1] JP 2004-20767A